SLERP (Spherical Linear-Interpolate) between two vectors from Python
Blender 2.71 (or any recent build) has Vector.slerp (spherical linear interpolation) which accepts values outside 0-1

So you can do:

a = Vector((1, 0, 0))
b = Vector((0, 1, 0))
c = a.slerp(b, 0.5)  # half way


To over rotate 10% do:

c = a.slerp(b, 1.1)




Note, this feature is not included in a stable release yet (but 2.71 will include it).